Optical module installing a semiconductor optical amplifier

ABSTRACT

An optical module providing a semiconductor optical amplifier (SOA), and a process to assembly the optical module are disclosed. The optical module provides front and rear coupling units each optically coupled with the SOA and fixed to the housing enclosing the SOA. The housing has a slim wall fixing a lens holder soldered to the slim wall. The front and/or rear coupling unit is fixed to the lens holder by YAG laser welding after the active alignment by using a spontaneous emission of the SOA, and amplified emission of externally provided test beam.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.14/794,387, filed Jul. 8, 2015, which claims the benefit of JapanesePatent Application No. 2014-141632, filed Jul. 9, 2014 and JapanesePatent Application No. 2014-141635, filed Jul. 9, 2014.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical module and a process toassemble the same. In particular, the present invention relates to anoptical module that provides a semiconductor optical amplifier (SOA),and a process to assemble the optical module.

2. Background Arts

An optical module providing an SOA therein has been well known in thefield of the optical communication, or the optical component. Such anoptical module often installs an optical isolator to prevent light fromreturning back to the SOA because the returned light causes opticalnoises. The SOA is necessary to be coupled with other optical devices orcomponents in respective facets thereof opposite to each other. When theoptical isolator couples with one of facets of the SOA, the activealignment of the other optical devices/components sometimes becomesimpossible because the optical isolator cuts light coming from the SOA.

Also, when an optical module implementing an SOA assembles other opticalcomponents as optically coupling with respective facets of the SOA,simplified processes and arrangements for assembling other componentswith the SOA are preferable because two facets arrangements sometimescause failures and easily brings miss-alignment between components. Thepresent invention is to solve those subjects.

SUMMARY OF THE INVENTION

An aspect of the present application relates to a process to assemble anoptical module. The optical module of the present application providesan amplifier unit and a coupling unit. the amplifier unit installs asemiconductor optical amplifier (SOA) therein. The coupling unitreceives an external optical fiber optically coupled with the SOA. Theprocess of the present application includes steps of: (1) activating theSOA to generate a spontaneous emission therefrom; and (2) aligning thecoupling unit with the SOA at a position where the spontaneous emissionof the SOA detected through the external optical fiber becomes a maximumor exceeds a preset threshold.

The optical module may further include another coupling unit thatprovides an optical isolator for cutting light coming from the SOA butpassing light going to the SOA. The process of the present applicationmay further provide steps of, after the step of aligning the couplingunit with the SOA, (3) providing a test beam to the SOA through theanother coupling unit; and (5) aligning the another coupling unit withthe SOA at a position where the test beam amplified by the SOA anddetected through the external fiber becomes a maximum or exceeds apreset threshold.

Another aspect of the preset application relates to an arrangement of anoptical module. The optical module comprises an amplifier unit, a lensholder, and a coupling unit. The amplifier unit may include a housingthat encloses a semiconductor optical amplifier (SOA) therein. The lensholder secures a lens that is optically coupled with the SOA. The lensholder may be fixed to the housing by soldering. The coupling unitreceives an optical fiber therein. A feature of the optical module isthat the coupling unit is welded to the lens holder but the lens holderis soldered to the housing.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other purposes, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 shows a cross section of an optical module according to the firstembodiment of the present application;

FIG. 2 is a flow chart of a process to assemble the optical module shownin FIG. 1;

FIG. 3 shows a process to assemble the front lens holder with thehousing of the amplifier unit;

FIG. 4 shows a process to install an intermediate assembly including theSOA, the front and rear collimating lenses, the thermistor, where theyare mounted on the thermos-electric cooler, into the housing of theamplifier unit;

FIG. 5 shows a process to align the rear coupling unit with the SOA andfix the rear coupling unit to the housing by YAG laser beam;

FIG. 6 shows a process to align the front coupling unit with the SOA andfix the front coupling unit to the housing by YAG laser beam;

FIG. 7 shows a cross section of an optical module according to thesecond embodiment of the present application;

FIG. 8 is a flow chart of a process to assemble the optical module shownin FIG. 7;

FIG. 9 shows a process to fix the front and rear lens holders to thehousing of the amplifier unit by soldering;

FIG. 10 shows a process to install an intermediate assemble includingthe SOA within the housing;

FIG. 11 shows a process to align the rear coupling unit with the SOA andfix the rear coupling unit to the rear lens holder by YAG laser beam;

FIG. 12 shows a process to align the front coupling unit with the SOAand fix the front coupling unit to the front lens holder by YAG laserbeam;

FIG. 13 shows a cross section of an optical module according to thethird embodiment of the present application;

FIG. 14 shows a process to fix the front lens holder to the housingaccording to the fourth embodiment of the present application;

FIG. 15 shows a process to fix the front and rear lens holders to thehousing according to the fourth embodiment of the present application;

FIG. 16 shows a cross section of an optical module according to thefourth embodiment of the present application;

FIG. 17 shows a cross section of an optical module according to thefifth embodiment of the present application; and

FIG. 18 shows a cross section of an optical module according to thesixth embodiment of the present application.

DESCRIPTION OF EMBODIMENTS

Next, some embodiments according to the present application will bedescribed as referring to drawings. In the description of the drawings,numerals or symbols same with or similar to each other will refer toelements same with or similar to each other without duplicatingexplanations.

First Embodiment

FIG. 1 illustrates across section of an optical module 1 assembled by aprocess according to an embodiment of the present invention. The opticalmodule 1 provides an amplifier unit 10, a front lens holder 21, a frontcoupling unit 22, and a rear coupling unit 32. In the description below,the direction “front” means a side where the front coupling unit 22 isassembled with respect to the amplifier unit 10, and the other direction“rear” means the side where the rear coupling unit 32 is assembled.However, these directions of the “front” and “rear” are only for theexplanation sake and may not affect the scope of the invention. Thefront lens holder 21 secures a lens 23 which optically couples with thefront coupling unit 22 and a semiconductor optical amplifier (SOA) 41enclosed in the amplifier unit 10. FIG. 1 also illustrates a frontoptical axis Ax1 connecting the front coupling unit 22 with the SOA 41and a rear optical axis Ax2 connecting the rear coupling unit 32 alsowith the SOA 41.

The amplifier unit 10 provides a housing 11 that includes side walls, 11h to 11 k. The front wall 11 h provides a front surface 11A and a frontopening 12. The front lens holder 21 provides a front surface 21A, and arear surface 21A, where the front surface 21A is fixed to the rearsurface 22B of the front coupling unit 22 and the rear surface 21B isfixed to the front surface 11A of the housing 11. The housing 11 may bemade of alloy containing iron (Fe), nickel (Ni), and cobalt (Co), whichis often called as Kovar. The front lens holder 21 and the frontcoupling unit 22 may be made of stainless steel and have respectivecylindrical shapes.

The housing 11 further provides a rear surface 11B and a rear opening 13in the rear wall 11 k. The rear opening 13 secures a lens 33, which is aconcentrating lens in the present embodiment, for coupling the SOA 41optically with the rear coupling unit 32. The rear coupling unit 32,which may have a cylindrical shape, provides a front surface 32A fixedto the rear surface 11B of the housing 11. The rear coupling unit 32,which may be made of stainless steel, receives an external fiber 37 andprovides a stub 34 therein.

The housing 11, in addition to the SOA 41, further encloses asub-carrier 42, front and rear collimating lenses, 43A and 43B, athermistor 45, a carrier 36, and a thermo-electric cooler (TEC) 47. TheSOA 41 may be made of semiconductor materials involved in a group ofInP, which means that lattice constants of such semiconductor materialsare substantially equal to or within a range capable of beingepitaxially grown on the InP substrate. Specifically, the SOA 41provides, on the n-type InP substrate, an n-type InP cladding layer, anactive layer comprising plurality of well layers made of InGaAsP and aplurality of barrier layers also made of InGaAsP alternately stacked toeach other, and a p-type InP cladding layer. The SOA 41, which isassembled between the front and rear collimating lenses, 43A and 43B,has a front facet 41A optically coupled with the front collimating lens43 and a rear facet 41B coupled with the rear collimating lens 44, whereeach of the facets, 41A and 41B, is set inclined to respective opticalaxes, Ax1 and Ax2, to suppress light output from the SOA 41 fromreturning the SOA 41.

The SOA 41 is mounted on the TEC 47 through the carrier 46 and thesub-carrier 42; while, the collimating lenses, 43A and 43B, and thethermistor 45 are mounted directly on the carrier 46. The TEC 47 may bea type of Peltier devices. The thermistor 45, which may sense atemperature of the SOA 41, is preferably set immediate to the SOA 41.

The housing 11 may further provide an electrical terminal 51 that mountselectrodes, 52 a to 52 f, and lead pins, 53 a to 53 b, thereon. Theelectrodes, 52 a to 52 f, are electrically connected to the SOA 41, thethermistor 45, and the TEC 47 inside of the housing 11; but, in theoutside of the housing, they are electrically connected to respectivelead pins, 53 a to 53 f. Thus, the electrodes, 52 a to 52 f, pass theside wall 11 j. The devices within the housing 11 may be controlledexternally through the lead pins, 53 a to 53 f, and the electrodes, 52 ato 52 f. The terminal 51 may be made of multi-layered ceramics. The leadpins, 53 a to 53 f, and the electrodes, 52 a to 52 f, are made of copper(Cu) based metal plated with gold (Au).

The front coupling unit 22 may be a type of an optical receptacle thatpluggably receives an external optical fiber, which is not illustratedin the figures, or a type of a pig-tailed coupling which permanentlyprovides an external optical fiber to be coupled with another opticalfiber through an optical connector provided in the outside of the frontcoupling unit 22. In the receptacle arrangement, the front coupling unit22 may provide a stub 24 and an optical isolator 25 attached to an endof the stub 24. The stub 24, which may be made of ceramics and providesa coupling fiber in a center thereof, abuts against a ferrule secured inan end of the external optical fiber. Abutting the ferrule with the tipof the external optical fiber against the stub 24, in particular, thetip of the coupling fiber secured in the center of the stub 24, theexternal optical fiber may be physically in contact with the couplingfiber, which is often called as the physical contact (PC), withoutcausing Fresnel reflection between the external optical fiber and thecoupling fiber. Thus, the external optical fiber may realize a stableoptical coupling against the lens front 23, or the SOA 41 in theamplifier unit 10. The optical isolator 45 passes light provided fromthe external optical fiber toward the SOA 41 but cuts the light providedfrom the SOA 41.

The optical module 1 of the present embodiment shown in FIG. 1 providesthe front wall 11 h, which has the front surface 11A, having a thicknessdifferent from that of the rear wall 11 k having the rear surface 11B.Specifically, the thickness of the front wall 11 h is smaller than thethickness of the rear wall 11 k, also smaller than the thickness ofother side walls, 11 i and 11 j, where the latter side wall 11 jprovides the terminal 51. The present housing 11 has the thickness W₁ inthe front wall 11 h of, for instance, 0.3 mm and thicknesses, W₂ and W₅,in the rear wall 11 k and the side wall 11 j of 1.0 mm, respectively.The front wall 11 h preferably has a thickness W₁ of 0.1 to 0.3 mm andthe rear wall 11 k has a thickness W₂ of greater than 0.8 mm. A wallwith a thickness greater than 0.8 mm may show tolerance against thelaser welding.

FIG. 2 is a flow chart of a process to assemble the optical module 1shown in FIG. 1. The process comprises steps S11 to S14, and FIGS. 3 to6 shows respective steps, S11 to S14.

As shown in FIG. 3, the process at step S1 according to the presentembodiment first assembles the front lens holder 21 with the housing 11,where the housing 11 assembles the terminal 51 in the side wall 11 j andthe rear opening 13 secures the lens 23 in advance to the assembly ofthe lens holder 21. The fixing of the front lens holder 21 may beperformed by soldering as heating the rear surface 21B of the lensholder 21 by, for instance, the induction heating. The induction heatingmay melt the solder applied on the rear surface 21B of the lens holder21, which may fix the lens holder 21 to the front surface 11A of thehousing 10.

Next, the process installs the TEC 47 within the housing 11 at step S12,where the TEC 47 mounts the sub-carrier 42 that assembles the SOA 41thereon, the front and rear collimating lenses, 43A and 43B, and thethermistor 45 through the carrier 46.

Then, the process assembles the rear coupling unit 32 with the amplifierunit 10 at step S13, as shown in FIG. 5. Specifically, the SOA 41 ispractically activated by providing a bias current thereto and anemission L1, which is a type of the spontaneous emission, is output fromthe rear facet 41B and collimated by the rear collimating lens 43B. Therear coupling unit 32 is aligned on the rear surface 11B of the housing10 such that the emission L1 thus provided from the SOA 41 opticallycouples with the optical fiber 37. That is, as the spontaneous emissionof the SOA 41 is detected at the other end of the optical fiber 37through the stub 34 of the rear coupling unit 32, the rear coupling unit32 in the front surface 32A, thereof is slid on the rear surface 11B ofthe housing 11, and the rear coupling unit 32 is fixed to the housing 10by, for instance, the YAG laser welding in a position at which thedetected spontaneous emission of the SOA 41 through the external opticalfiber 37 becomes a maximum, or greater than a preset threshold. The YAGlaser beams are irradiated concentrically on the corners between thehousing 10 and the rear coupling unit 32 to perform the fillet-welding.

Subsequently, the process assembles the front coupling unit 22 with thefront lens holder 21 at step S14. Because the front coupling unit 22implements an optical isolator 25 therein, the spontaneous emission L1of the SOA 41 may be hard to detect through the front coupling unit 22.Accordingly, the process according to the present embodiment externallyprovides a test beam L2 to the SOA 41 through the front coupling unit22. The test beam L2 thus provided may be amplified by the SOA 41 andthe amplified beam L1, which may be a spontaneous emission or astimulated emission of the SOA 41, is collimated by the rear collimatinglens 43B, concentrated on the end of the stub 34 by the concentratinglens 33, and detected through the optical fiber 37 coupled with the stub34. Similar to the aforementioned step S13, the front coupling unit 22in the rear surface 22B thereof is welded by the YAG laser beam to thefront surface 21A of the front lens holder 21 at a position where theamplified test beam L2 detected through the optical fiber 37 becomes amaximum or greater than a preset reference.

Advantages of the process to assemble the optical module 1 according tothe present embodiment will be described. The process first fixes thefront lens holder 21 to the front surface 11A of the housing 11 bysoldering, which enables the front lens holder 21 to be easily fixed tothe housing 11 even when the thickness of the front wall 11 h isrelatively slim. The YAG laser beam sometimes causes open holes for sucha slim wall, or results in unreliable welding. A soft fixing such assoldering may fix two metal members without accompanying with any openholes. Moreover, the present embodiment uses the induction heating tomelt the soldering, which realizes a uniform melting of the solder andfixing two metal members. The YAG laser welding fixes the front couplingunit 22 with the front lens holder 21, because the front lens holder 21has a substantial width or thickness to secure the front lens 23.Accordingly, the YAG laser welding causes no open holes in the frontlens holder 21, or the front coupling unit 22.

The front coupling unit 22 provides the optical isolator 25 therein,which may effectively suppress or substantially prevent the light fromoutgoing to the optical coupling unit 22 from the SOA 41. For instance,when the optical module 1 receives light generated from a semiconductorlaser diode (LD) provided in a far end of the external optical fiber setin the front coupling unit 22, the LD may be effectively protected fromreceiving backward light that becomes optical noise sources anddrastically degrades the performance of the LD.

When the front coupling unit 21 provides an optical isolator 25, theoptical alignment, namely, the active optical alignment, becomes hardbecause the optical isolator 25 cuts light outgoing from the SOA 41,where the light is to be available for the active optical alignment. Thepresent optical module 1 firstly performs the optical alignment for therear coupling unit 32 by the spontaneous emission from the SOA 41, then,performs the alignment for the front coupling unit by providing the testbeam L2 to the SOA through the front coupling unit 22 and detecting theamplified test beam L2 output through the rear coupling unit 32. Thus,the front and rear coupling units, 22 and 32, may be optically alignedwith the SOA 41 by the active alignments even the optical isolator 25 isput in front of the SOA 41.

Second Embodiment

FIG. 7 shows a cross section of an optical module 1P according to thesecond embodiment of the present application. The optical module 1P ofthe second embodiment has a feature distinguishable from the opticalmodule 1 of the first embodiment that the housing 11P of the amplifierunit 10P has a rear wall 11 n, which includes the rear opening 13 andthe rear surface 11D, different from the rear wall 11 k of theaforementioned embodiment. Other arrangements and configurations of theoptical module 1P are the same with, or substantially same with those ofthe optical module 1 of the first embodiment.

The housing 11P of the amplifier unit 10P of the second embodimentprovides the rear wall 11 n having a thickness thereof nearly equal tothe thickness of the front wall 11 h and no lens within the opening 13thereof. Substituting the thicker rear wall 11 k of the firstembodiment, the optical module 1P of the second embodiment provides therear lens holder 31 that secures the lens 33 in a bore 31 c thereof. TheSOA 41 in the rear facet 41B thereof optically couples with the rearcoupling unit 32 through the rear lens holder 31. The rear lens holder31 provides the front and rear walls, 31A and 31B, where the formersurface 31A is fixed to the rear wall 13D of the housing 11P, while, therear coupling unit 32 in the front surface 32A thereof is fixed to therear surface 31B of the rear lens holder 31. The rear lens holder 31 maybe made of, similar to the front lens holder 21, stainless steel ormaterial having wettability for solder.

FIG. 8 is a flow chart of the process to assemble the optical module 1Pshown in FIG. 7, and FIGS. 9 to 12 show respective steps S21 to S24 ofthe process exhibited in FIG. 8.

The process of the second embodiment first assembles the front lensholder 21 with the housing 11P at step S21. That is, after fixing theterminal 51 in the side wall 11 j, applying solder on the rear surface21B of the front lens holder 21 and the front surface 11A of the housing11P, the front lens holder 21 is fixed to the front surface 11A of thehousing 11P by melting the solder on both surfaces, 21B and 11A, thensolidifying the solder.

Then, the process installs an assembly that includes the TEC 47 mountingthe SOA 41 on the sub-carrier 42, the front and rear collimating lenses,43A and 43B, and the thermistor 45 through the carrier 46 at step S22.After the installation of the assembly, the electrodes, 52 a to 52 f, onthe terminal 51 are connected to respective devices, 41, 45, and 47.

Next, the process assembles the rear lens holder 31 and the rearcoupling unit 32 with the housing 11P at step S23. The rear lens holder31 is first fixed to the housing 11P by, for instance, soldering.Specifically, similar to the fixing of the front lens holder 21, firstapplying solder on the rear surface 11D of the housing 11P and the frontsurface 31A of the rear lens holder, then melting and solidifying thesolder, the housing 11P is fixed to the rear lens holder 31.

Subsequently, the rear coupling unit 32 is fixed to the rear lens holder31. Similar to the process of the first embodiment, the SOA 41 ispractically activated by supplying a bias current to generate aspontaneous emission L1 from the rear facet 41B. The emission L1 iscollimated by the rear collimating lens 43B and concentrated by the lens33 onto the end of the external fiber 37, or an end of the stub 34secured in the rear coupling unit 32. The rear coupling unit 32 is fixedin a position at which the spontaneous emission L1 detected through theexternal fiber 37 becomes a maximum or greater than a preset level. Therear coupling unit 32 is welded to the rear lens holder 31 at theposition by the YAG laser beam.

Finally, the front coupling unit 22 is assembled with the housing 11P byprocesses same with those of the first embodiment. Specifically,supplying a test beam L2 to the SOA 41 through an external fiber, whichis not shown in FIG. 12, and the front lens 21, and detecting theamplified test beam L2 at the end of the external fiber 37, the frontcoupling unit 22 is aligned in a position where the detected test beamL2 becomes a maximum. The front coupling unit 22 is welded to the frontlens holder 21 at the aligned position.

The optical module 1P of the second embodiment, thus described in theassembling process thereof, provides the amplifier unit 10P whosehousing 11P includes the rear wall 11 n with a thickness ofsubstantially equal to that of the front wall 11 h to which the frontlens holder 21 is fixed. The optical module 1P further provides the rearlens holder 31 to secure the rear concentrating lens 33 in the bore 31 cthereof, and the rear coupling unit 32 welded to the rear lens holder 31not the rear wall 11 n of the housing 11P. The front wall 11 h and therear wall 11 n have the thicknesses, W₁ and W₄, of 0.3 mm, and the sidewall 11 j has the thickness of 0.8 mm. The front and rear walls, 11 hand 11 n, preferably have respective thicknesses of 0.1 to 0.3 mm,while, the side wall 11 j preferably has a thickness greater than 0.8mm. Accordingly, even the housing 11P of the amplifier unit 10P providesa slim rear wall 11 n, which is unable to secure the rear concentratinglens therein and to fix the rear coupling unit 32 by welding, the rearcoupling unit 32 is fixed by welding through the rear lens holder 31.

Also, even when the optical module 1P installing the SOA 41 therein isnecessary to align optical units in both of front and rear of the SOA41, and one of the optical units installs an optical isolator therein,which is unable to extract the test beam for the optical alignment outof the optical module, the active alignment of the front coupling unit22 becomes possible by operating the SOA 41 in the amplifying mode.

Third Embodiment

FIG. 13 shows a cross section of an optical module 1Q according to stillanother embodiment of the present application. The optical module 1Q ofthe third embodiment has a feature distinguishable from those of theaforementioned embodiments in a point where the front coupling unit 22Qprovides an optical fiber 27 therein. Other arrangements orconfigurations of the optical module 1Q are same with or substantiallysame with those of the optical modules, 1 and 1P, of the aforementionedembodiments.

Because the front coupling unit 22Q of the present embodiment providesno optical isolator; the process may perform the active alignment forthe front coupling unit 22Q by the spontaneous emission caused in theSOA 41. Specifically, the SOA 41 causes an emission by being suppliedwith a bias current. The emission L3 is able to be extracted through thefront lens 23 and the optical fiber 27 in the front coupling unit 22Q.The front coupling unit 22Q is fixed to the front lens holder 21 by theYAG laser welding at a position where the emission L3 detected throughthe optical fiber 27 becomes a maximum, which is similar to thealignment and the fixing of the rear coupling unit 32 with respect tothe rear lens holder 31.

Thus, the process of the present embodiment performs the activealignment for the front and rear coupling units, 22Q and 32, by usingthe spontaneous emissions, L3 and L1, from the SOA 41, which simplifiesthe process to assemble the optical module.

Fourth Embodiment

FIGS. 14 to 16 show cross sections of an optical module 1S still anotherembodiment of the present application. The optical module 1S of thepresent embodiment has a feature distinguishable from aforementionedembodiments in a point where the front and rear lens holders, 121 and131, provide flanges, 21 d and 31 d around the bore 21 c and 31 cthereof fit within respective openings, 12 s and 13 s, of the housing11S. The outer dimensions or diameter of the flanges, 21 d and 31 d, areslightly smaller than diameters of edges, 12 d and 13 d, of theopenings, 12 s and 13 s. Accordingly, fitting the flanges, 21 d and 31d, into respective openings, 12 s and 13 s, the positions of the lensholders, 21 and 31, against the housing 11S may be automaticallydetermined.

This means that, the lens holders, 121 and 131, secures respectiveconcentrating lenses, 23 and 33, in a center of the bores, 21 c and 31c. The fitting of the flange 21 d and 31 d into the openings, 12 s and13 s, the center of the lenses, 23 and 33, may be determined. The lensholders, 121 and 131, may be fixed to the housing 11S by soldering.Specifically, applying the solder on the rear surface 21B of the lensholder 121 in a portion around the flange 21 d but not on the side andthe top of the flange 21 d, also applying solder on the front surface11A of the housing 11S but not on the edge 12 d of the opening 12 s,fitting the lens holder 121 into the opening 12 s, and melting thesolder by, for instance, the induction heating and solidifying thesolder, the lens holder 121 is assembled with the housing 11S. Theprocess to fix the front lens holder 121 to the housing 11S is shown inFIG. 14.

FIG. 15 shows a process to assemble the front and rear lens holders, 121and 131, to the housing 11S. As shown in FIG. 15, the optical module 1Smay provide not only the front lens holder 121 but the rear lens holders131 each having the flanges, 21 d and 31 d, fitting in the front andrear openings, 12 s and 13 s, of the housing 11S. Because both the frontand rear lens holders, 121 and 131, provide the flanges, 21 d and 31 d,the alignment of the lenses, 23 and 33, with respect to the housing 11S.That is, inserting the flanges, 21 d and 31 d, into the openings, 12 sand 13 s, the lenses, 23 and 33, may be roughly aligned in respectivecenters of the openings, 12 s and 13 s, of the housing 11S.

FIG. 16 shows a cross section of the optical module 1S according to thefourth embodiment of the present application. The optical module 1Sprovides the front and rear lens holders, 121 and 131, thus describedbased on FIGS. 14 and 16. Arrangements or configurations except for thefront and rear lens holders, 121 and 131, are same with those ofaforementioned embodiments. Each of the front and rear lens holders, 121and 131, are fit into respective openings, 12 s and 13 s, of the housing11S, which automatically aligns the center of the lenses, 23 and 33,against the housing 11S. The process to assemble the optical modulesinstalls the assembly including the SOA 41 and the front and rearcollimating lenses, 43A and 43B, into the housing after the fixing ofthe lens holder as aligning the SOA 41 with respect to the front andrear concentrating lenses, 23 and 33. When the optical axis of thelenses, 23 and 33, that is, the lenses, 23 and 33, in the optical axesthereof are considerably offset from the designed axes, the front andrear coupling units, 22 and 32, become unable to couple with the SOA 41,or to extract the light by substantial magnitude. The arrangement of thelens holder, 121 and 131, having the flange, 21 d and 31 d, may alignthe lens, 23 and 33, with the opening, 12 s and 13 s, and acceleratesthe alignment of the SOA 41 with respect to the housing.

Fifth Embodiment

FIG. 17 shows a cross section of an optical module 1U according to stillanother embodiment of the present application. The optical module 1U hasa feature distinguishable from those of the aforementioned embodimentsin that, not only the front coupling unit 22, but the rear coupling unit132 provides an optical isolator 35. Other arrangements except for therear optical isolator 35 are substantially same with those of theaforementioned embodiments.

The optical isolator 35 may pass light coming from the SOA 41 butsubstantially cut light going to the SOA 41. Accordingly, the rearoptical isolator 35 causes no effect for the active alignment of therear coupling unit 132 with the SOA 41 and that of the front couplingunit 22 with the SOA 41. That is, the active alignment of the rearcoupling unit 132 may utilize the spontaneous emission L1 of the SOA 41,and the active alignment of the front coupling unit 22 may utilize thetest beam provided externally to the front coupling unit 22 and theamplified test beam provided from the SOA 41 through two lenses, 43B and33, and the optical fiber 37. The rear coupling unit 132 and the frontcoupling unit 22 are fixed to the respective position with respect tothe housing 11P of the amplifier unit 10P where the spontaneous emissionL1 and the amplified test beam L2 in the magnitudes thereof becomerespective maxima.

Sixth Embodiment

FIG. 18 shows a cross section of an optical module 1T according to thesixth embodiment of the present application. The optical module 1T shownin FIG. 18 has a feature distinguishable from those of aforementionedembodiments in that the housing 11T optical amplifier unit 10T providesa thick front wall 11 m and omits the front lens holder 21 to secure thefront lens 23. Instead, the thick front wall 11 m holds the front lens23 same as those of the rear wall 11 k. Other arrangements of theoptical module 1T except for the front wall 11 m and no front lensholder 21 are same with those of the optical module 1 of the firstembodiment shown in FIG. 1.

The process to assemble the optical module 1T fundamentally same withthe process for the optical module 1 of the first embodiment except forthat the assembly installed within the housing 11T of the amplifier unit10T is set after building the terminal 51 in the side wall 11 j butbefore assembling of the rear coupling unit 32 with the housing 11T.That is, the optical module 1T may omit the step to solder the frontlens holder 21 to the front surface 11A of the housing 11T. The rearcoupling unit 32 is aligned and fixed by the YAG laser welding to thehousing 11T at the position where the spontaneous emission of the SOA 41in the magnitude thereof becomes a maximum as detecting the emissionoutput from the rear facet 41B trough two lenses, 43B and 33, and theoptical fiber.

Also, in the process for the optical module 1T, the front coupling unit22 is aligned with the SOA 41 on the front surface 11A of the housing11T not the front surface 21A of the front lens holder 21, and directlywelded to the frond surface 11A of the housing 11T by the YAG laserbeam. Specifically, providing the test beam L2 to the SOA through theoptical fiber set within the front coupling unit 22 and two lenses, 23and 43A, amplifying the test beam L2 by the SOA 41, and detecting theamplified test beam L2 output from the rear facet 41B of the SOA 41through two lenses, 43B and 33, and the other optical fiber, the frontcoupling unit 22 is fixed at the position where the thus detected testbeam L2 becomes a maximum.

Thus, the process to assemble the optical module 1T may show advantagessame with those of the aforementioned embodiment. That is, the rearcoupling unit 32 is actively aligned as using the spontaneous emissioncoming from the SOA 41, and the front coupling unit 22 is also activelyaligned as using the test beam external provided to the optical module1T and amplified by the SOA 41 internally installed within the amplifierunit 10T.

In the foregoing detailed description, the process and module accordingto the present invention have been described with reference to specificexemplary embodiments thereof. It will, however, be evident that furthermodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

I claim:
 1. An optical module, comprising: an amplifier unit including ahousing that encloses a semiconductor optical amplifier (SOA) therein,the housing having a front wall and a side wall; a lens holder thatsecures a lens optically coupled with the SOA, the lens holder beingsoldered to the front wall of the housing; and a coupling unit thatreceives an optical fiber therein, the coupling unit being welded to thelens holder, wherein the front wall has a thickness smaller than athickness of the side wall.
 2. The optical module of claim 1, whereinthe housing further includes a rear wall opposite to the front wall, therear wall having a thickness smaller than a thickness of the side wall,wherein the optical module further includes another lens holder solderedto the rear wall.
 3. The optical module of claim 2, wherein the anotherlens holder secures another lens therein.
 4. The optical module of claim1, wherein the coupling unit includes an optical isolator that passeslight going to the SOA but cuts another light coming from the SOA. 5.The optical module of claim 1, wherein the front wall has a thicknessless than a half of a thickness of the side wall.